Display device and method of driving a display device

ABSTRACT

Write in of lower significant bits of a digital video signal to a memory is eliminated by a memory controller of a signal control circuit in a display device during a second display mode in which the number of gray scales is reduced, as compared to a first display mode. Further, read out of the lower significant bits of the digital video signal from the memory is also eliminated. The amount of information of digital image signals input to a source signal line driver circuit is reduced. Corresponding to this operation, a display controller functions to make start pulses and clock pulses input to each driver circuit have a lower frequency, and write in periods and display periods of sub-frame periods participating in display are set longer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/276,212, filed Feb. 17, 2006, now allowed, which is a continuation ofU.S. application Ser. No. 10/118,917, filed Apr. 10, 2002, now U.S. Pat.No. 7,027,074, claims the benefit of a foreign priority applicationfiled in Japan as Serial No. 2001-121883 on Apr. 20, 2001, all of whichare incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device for displaying animage by inputting a digital video signal. In particular, the presentinvention relates to a display device having light emitting elements.Further, the present invention relates to an electronic equipment thatuses the display device.

2. Description of the Related Art

A display device having a light emitting element disposed in each pixelwhich performs display of an image by controlling light emitted from thelight emitting elements is explained below.

The explanation throughout this specification uses elements (OLEDelements) having a structure in which an organic compound layer foremitting light when an electric field is generated is sandwiched betweenan anode and a cathode, for the light emitting elements, but the presentinvention is not limited to this structure.

Further, the explanation within this specification uses elements thatutilize light emitted when making a transition from singlet excitons toa base state (fluorescence), and those that utilize light emitted whenmaking a transition from triplet excitons to a base state(phosphorescence).

Layers such as hole injecting layers, hole transporting layers, lightemitting layers, electron transporting layers, electron injecting layerscan be given as organic compound layers. Light emitting elementsbasically are shown by structures in which an anode, a light emittinglayer, and a cathode overlap in this order. In addition, structures suchas a structure in which an anode, a hole injecting layer, a lightemitting layer, an electron injecting layer, and a cathode areoverlapped in this order, and one in which an anode, a hole injectinglayer, a hole transporting layer, a light emitting layer, an electrontransporting layer, an electron injecting layer, and a cathode areoverlapped in this order may also be used.

Note that the organic compound layers are not limited to laminatestructures in which layers such as hole injecting layers, holetransporting layers, light emitting Layers, electron transportinglayers, and electron injecting layers are clearly separated from eachother. That is, the organic compound layers may also have a structurehaving a layer in which the materials used for structuring holeinjecting layers, hole transporting layers, light emitting layers,electron transporting layers, and electron injecting layers are mixed.

Further, any types of materials of low molecular weight materials, highmolecular weight materials, and intermediate molecular weight materialsmay be used as the OLED element organic compound layers.

Note that, in this specification, the term intermediate molecular weightmaterial indicates materials having a molecularity equal to or less than20, or those in which the length of the chained molecules is equal to orless than 10 μm and which do not have sublimation property.

A display devices is structured by a display and peripheral circuits forinputting signals to the display.

The structure of the display is explained below.

The display is structured by a source signal line driver circuit, a gatesignal line driver circuit, and a pixel portion. The pixel portion haspixels disposed in a matrix shape.

Thin film transistors (hereafter referred to as TFTs) are arranged ineach pixel of the pixel portion. A method of placing two TFTs in eachpixel and controlling light emitted from the light emitting element ofeach pixel is explained.

FIG. 7 shows a structure of a pixel portion of a display device.

Source signal lines S1 to Sx, gate signal lines G1 to Gy, and electricpower source supply lines V1 to Vx are arranged in a pixel portion 700,and x columns and y rows (where x and y are natural numbers) of pixelsare also placed in the pixel portion. Each pixel 800 has a switching TFT801, a driver TFT 802, a storage capacitor 803, and a light emittingelement 804.

An enlarged view of one pixel of the pixel portion of FIG. 7 is shown inFIG. 8.

The pixel is structured by one source signal line S of the source signallines S1 to Sx, one gate signal line G of the gate signal lines G1 toGy, one electric power source supply line V of the electric power sourcesupply lines V1 to Vx, the switching TFT 801, the driver TFT 802, thestorage capacitor 803, and the light emitting element 804.

A gate electrode of the switching TFT 801 is connected to the gatesignal line G, and one of a source region and a drain region of theswitching TFT 801 is connected to the source signal line S, while theother one is connected to a gate electrode of the driver TFT 802 or toone electrode of the storage capacitor 803. One of a source region and adrain region of the driver TFT 802 is connected to the electric powersource supply line V, while the other one is connected to an anode or acathode of the light emitting element 804. The electric power sourcesupply line V is connected to one of the two electrodes of the storagecapacitor 803, namely the electrode on the side to which the driver TFT802 and the switching TFT 801 are not connected.

The anode of the light emitting element 804 is referred to as a pixelelectrode, and the cathode of the light emitting element 804 is referredto as an opposing electrode, within this specification for cases inwhich the source region or the drain region of the driver TFT 802 isconnected to the anode of the light emitting element 804. On the otherhand, if the source region or the drain region of the driver TFT 802 isconnected to the cathode of the light emitting element 804, then thecathode of the light emitting element 804 is referred to as the pixelelectrode, and the anode of the light emitting element 804 is referredto as the opposing electrode.

Further, an electric potential imparted to the electric power sourcesupply line V is referred to as an electric power source electricpotential, and an electric potential imparted to the opposing electrodeis referred to as an opposing electric potential.

The switching TFT 801 and the driver TFT 802 may be either p-channelTFTs or n-channel TFTs. However, it is preferable that the driver TFT802 be a p-channel TFT, and that the switching TFT 801 be an n-channelTFT for cases in which the pixel electrode of the light emitting element804 is the anode. Conversely, it is preferable that the driver TFT 802be an n-channel TFT, and that the switching TFT 801 be a p-channel TFTif the pixel electrode is the cathode.

Note that the storage capacitor 803 need not always be formed.

For example, a parasitic capacitance generally referred to as a gatecapacitance is formed in overlapping regions for cases where there is anLDD region in which the n-channel TFT used as the driver TFT 802 isformed so as to overlap with a gate electrode through a gate insulatingfilm. It is possible to actively use this parasitic capacitance as astorage capacitor for storing a voltage applied to the gate electrode ofthe driver TFT 802.

Operation during display of an image with the aforementioned pixelstructure is explained below.

A signal is input to the gate signal line G, and the electric potentialof the gate electrode of the switching TFT 801 changes, thereby changinga gate voltage. The signal is input to the gate electrode of the driverTFT 802 by the source signal line S, via the source and drain of theswitching TFT 801 which thus has been placed in a conductive state.Further, the signal is stored in the storage capacitor 803. The gatevoltage of the driver TFT 802 changes in accordance with the signalinput to the gate electrode of the driver TFT 802, thereby placing thesource and drain in a conductive state. The electric potential of theelectric power source supply line V is imparted to the pixel electrodeof the light emitting element 804 through the driver TFT 802. The lightemitting element 804 thus emits light.

A method of expressing gray scales with pixels having such a structureis explained.

Gray scale expression methods can be roughly divided into analog methodsand digital methods. Digital methods have advantages compared to analogmethods, such as being geared to multiple gray scales.

A digital gray scale expression method is focused upon here.

A time gray scale method can be given as the digital gray scaleexpression method.

A time gray scale driving method is explained in detail below.

The time gray scale driving method is a method of expressing gray scalesby controlling the period that each pixel of a display device emitslight.

If a period for displaying one image is taken as one frame period, thenone frame period is divided into a plurality of subframe periods.

Turn on and turn off, namely whether or not the light emitting elementof each pixel is made to emit light or to not emit light, is performedfor each subframe period. The period during which the light emittingelement emits light in one frame period is controlled, and a gray scalefor each pixel is expressed.

The time gray scale driving method is explained in detail using timingcharts of FIGS. 5A and 5B.

Note that an example of expressing gray scales using a 4-bit digitalimage signal is shown in FIG. 5A.

Note also that FIG. 7 and FIG. 8 may be referred to regarding thestructure of the pixel portion and the structure of the pixels,respectively.

In accordance with an external electric power source (not shown in thefigures), the opposing electric potential can be switched over betweenan electric potential on the same order as the electric potential of theelectric power source supply lines V1 to Vx (electric power sourceelectric potential), and an electric potential of the electric powersource supply lines V1 to Vx on an order sufficient to make the lightemitting element 804 emit light.

One frame period F is divided into a plurality of subframe periods SF1to SF4.

The gate signal line G1 is selected first in the first subframe periodSF1, and a digital image signal is input from the source signal lines S1to Sx to each of the pixels having the switching TFTs 801 with gateelectrodes connected to the gate signal line G1. The driver TFT 802 ofeach pixel is placed in an on state or an off state by the input digitalimage signal.

The term “on state” for a TFT in this specification indicates that theTFT is in a state in which there is conduction between the source andthe drain in accordance with a gate voltage. Further, the term “offstate” for a TFT indicates that there is a non-conductive state betweenthe source and the drain in accordance with the gate voltage.

The opposing electric potential of the light emitting elements 804 isset nearly equal to the electric potential of the electric power sourcesupply lines V1 to Vx (electric power source electric potential) at thispoint, and therefore the light emitting elements 804 do not emit lighteven in pixels having their driver TFT 802 in an on state.

FIG. 5B is a timing chart showing operation when the digital imagesignal is input to the driver TFT 802 of each pixel.

A sampling period in which a source signal line driver circuit (notshown in the figures) samples signals corresponding to each of thesource signal lines are shown by reference symbols S1 to Sx in FIG. 5B.The sampled signals are output at the same time to all of the sourcesignal lines in a return period in the figure. The signals thus outputare thus input to the gate electrodes of the driver TFTs 802 in thepixels which have selected gate signal lines.

The aforementioned operations are repeated for all of the gate signallines G1 to Gy, and a write in period Ta1 is completed.

Note that a period for write-in during the first subframe period SF1 iscalled Ta1. In general, a write in period of a j-th sub-frame period SFj(where j is a natural number) is called Taj.

The opposing electric potential changes when the write in period Ta1 iscomplete, so as to have an electric potential difference from theelectric power source electric potential on an order so that the lightemitting element 804 will emit light. A display period Ts1 thus begins.

Note that the display period of the first subframe period SF1 is calledTs1. In general, a display period of the j-th sub-frame period SFj(where j is a natural number) is denoted by using a reference symbolTsj.

The light emitting elements 804 of each pixel are placed in a lightemitting state or a non-light emitting state, corresponding to the inputsignal, in the display period Ts1.

As shown in FIG. 5A, the above operations are repeated for all of thesubframe periods SF1 to SF4, thereby completing one frame period F1.

The length of the display periods Ts1 to Ts4 of the subframe periods SF1to SF4 are set appropriately here, and gray scales are expressed by anaccumulation of the display periods of the subframe period during whichthe light emitting elements 804 emit light. In other words, the totalamount of the turn on time within one frame period is used to expressthe gray scales.

A method of generally expressing 2^(n) gray scales by inputting an n-bitdigital video signal, is explained.

One frame period is divided into n sub-frame periods SF1 to SFn at thispoint, for example, and the ratios of the lengths of the display periodsTs1 to Tsn of the sub-frame periods SF1 to SFn are set so as to be Ts1:: Ts2 :: ... :: Tsn-1 :: Tsn=2⁰ :: 2⁻¹ :: ... :: 2^(−n+2) :: 2^(−n+1).Note that the lengths of the write in periods Ta1 to Tan are all thesame.

Within one frame period, the gray scale of the pixels in the frameperiod is determined by finding the total of the display period Tsduring which a light emitting state is selected in the light emittingelements 804. For example, if the brightness for a case in which a pixelemits light during all of the display periods is taken to be 100% whenn=8, then a brightness of 1% can be expressed if the pixel emits lightin the display period Ts8 and in the display period Ts7. A 60%brightness can be expressed for cases in which the pixel emits light inthe display periods Ts6, Ts4, and Ts1.

A circuit for inputting a signal in order to perform the above-statedtime gray scale driving method to the source signal line driver circuitand the gate signal line driver circuit of the display is explainedusing FIG. 10.

Signals input to the display device are referred to as digital videosignals within this specification. Note that the example explained hereis that of a display device into which an n-bit digital video signal isinput.

The display device is structured by: a display 1100 composed of a sourcesignal line driver circuit 1107, a gate signal line driver circuit 1108,and a pixel portion 1109; a signal control circuit 1101; and a displaycontroller 1102.

The digital video signal is read in by the signal control circuit 1101,and the signal control circuit 1101 outputs a digital image signal (VD)to the display 1100.

A signal converted for input to the display 1100 in the signal controlcircuit, the edited digital video signal, is referred to as the digitalimage signal within this specification.

Signals for driving the source signal line driver circuit 1107 and thegate signal line driver circuit 1108 of the display 1100 are input fromthe display controller 1102.

The structure of the signal control circuit 1101 and the structure ofthe display controller 1102 are explained.

Note that the source signal line driver circuit 1107 of the display 1100is structured by a shift register 1110, an LAT (A) 1111, and an LAT (B)1112. In addition, although not shown in the figures, circuits such aslevel shifters and buffers may also be formed.

The signal control circuit 1101 is structured by a CPU 1104, a memory A1105, a memory B 1106, and a memory controller 1103.

The digital video signal input to the signal control circuit 1101 isinput to the memory A 1105 through the CPU 1104.

In other words, the digital signal for each bit, corresponding to eachpixel, in the digital video signal is input to the memory A 1105 andstored.

The memory A 1105 has a capacity that is capable of storing the n-bitdigital signal for all pixels of the pixel portion 1109 of the display1100.

When one frame period portion of the digital signal is stored in thememory A 1105, the digital signal for each bit is read out in order bythe memory controller 1103, and then input to the source signal linedriver circuit as the digital image signal VD.

The digital video signal corresponding to the next frame period is theninput to the memory B 1106, through the CPU 1104, when read out of thedigital signals stored in the memory A 1105 begins, and storage of thedigital video signal in the memory B begins. Similarly to the memory A1105, the memory B 1106 also has a capacity that is capable of storingthe n-bit digital signal for all pixels of the pixel portion of thedisplay device.

The signal control circuit 1101 thus has the memory A 1105 and thememory B 1106, each of which is capable of storing one frame periodportion of the n-bit digital signal. The digital video signal is sampledusing the memory A 1105 and the memory B 1106 alternately.

The signal control circuit 1101 for storing signals by using the twomemories alternately, namely the memory A 1105 and the memory B 1106, isshown here. In general, however, memories capable of storing informationcorresponding to a plurality of frame portions are used. These memoriescan be used alternately.

The structure of the memory controller 1103, used for controlling inputof the digital video signal to, and read out of the signals from, thememory A 1105 and the memory B 1106 of the signal control circuit 1101,is explained using FIG. 11.

In FIG. 11, the memory controller 1103 is structured by a memoryread/write control (hereafter referred to as memory R/W) circuit 1202, astandard oscillator circuit 1203, a variable frequency divider circuit1204, an x-counter 1205 a, a y-counter 1205 b, an x-decoder 1206 a, anda y-decoder 1206 b.

Both memories, namely the memory A and the memory B, of theaforementioned signal control circuit are hereafter taken together anddenoted as memory. Further, the memory is structured by a plurality ofmemory elements, and the memory elements are selected by using (x,y)addresses.

Signals from the CPU 1104 are input to the standard oscillator circuit1203. Signals from the standard oscillator circuit 1203 are input to thevariable frequency divider circuit 1204 and converted to signals havingan appropriate frequency. The signals from the variable frequencydivider circuit 1204 select x addresses of the memory through thex-counter 1205 a and the decoder 1206 a. At the same time, the signalsfrom the variable frequency divider circuit 1204 select y addresses ofthe memory through the y-counter 1205 b and the y-decoder 1206 b. Inthis way, the addresses of the memory (x, y) are selected. Furthermore,signals from the CPU 1104 are input to the memory R/W circuit 1202, anda memory R/W signal for selecting write in operation of the signal tothe memory, or read out operation of the signal from the memory, isoutput.

Memory addresses for writing in, or reading out, the digital signals arethus selected by the memory x address and the memory y address.Operations for write of the digital signal to, or read out of thedigital signal from, the memory element selected by this address areperformed in accordance with the memory R/W signal.

Next, the structure of the display controller 1102 in FIG. 10 isexplained below.

The display controller 1102 outputs signals such as start pulses (S_SP,G_SP) and clock pulses (S_CLK, G_CLK) to the source signal line drivercircuit 1107 and to the gate signal line driver circuit 1108.

The structure of the display controller 1102 is explained using FIG. 12.

The display controller 1102 is structured by a standard clock generatorcircuit 1301, a horizontal clock generator circuit 1303, a verticalclock generator circuit 1304, and an electric power source controlcircuit 1305 used for the light emitting elements.

A clock signal 31 input from the CPU 1104 is input to the standard clockgenerator circuit 1301, and a standard clock is generated. The standardclock is input to the horizontal clock generator circuit 1303 and to thevertical clock generator circuit 1304. Further, a horizontal periodsignal 32 for determining a horizontal period is input from the CPU 1104to the horizontal clock generator circuit 1303, and the clock pulseS_CLK and the start pulse S_SP used for the source signal line drivercircuit are output. At the same time, a vertical period signal 33 fordetermining a vertical period is input from the CPU 1104 to the verticalclock generator circuit 1304, and the clock pulse G_CLK and the startpulse G_SP used for the gate signal line driver circuit are output.

FIG. 10 will be referred to again.

The start pulse S_SP and the clock pulse S_CLK output from the displaycontroller 1102 and used for the source signal line driver circuit areinput to the shift register 1110 of the source signal line drivercircuit 1107 in the display 1100. Further, the start pulse G_SP and theclock pulse G_CLK used for the gate signal line driver circuit are inputto the gate signal line driver circuit 1108 of the display 1100.

In the display controller 1102, the electric power source controllercircuit 1305 used for the light emitting element maintains the electricpotential of the opposing electrode of the light emitting element ofeach pixel in the display at the same electric potential as the electricpower source electric potential during the write in period. Further, theelectric power source controller circuit 1305 controls the electricpotential of the opposing electrode so that it changes to have anelectric potential difference, with respect to the electric power sourceelectric potential, on an order such that the light emitting elementemits light.

The display device thus displays an image.

It is preferable that the display device have as little electric powerconsumption as possible here. Low electric power consumption isespecially desirable if the display device is incorporated into aportable information device or the like to be utilized.

A method of suppressing the electric power consumption of the displaydevice by reducing the number of gray scales during image display (thenumber of gray scales expressed) in the case in which a multiple grayscale display is not required, is proposed.

This method is explained in detail below using a timing chart of FIG. 9.

A display device, into which a 4-bit signal is input to thereby display2⁴ gray scales, is noticed. Gray scales are expressed by using only themost significant 1-bit signal (digital signal) in accordance with aswitching signal. A method of reducing the electric power consumption ofthe display device is explained here in an example.

A case of inputting a 4-bit digital video signal and expressing 2 ⁴ grayscales is referred to as a first display mode, and a case of expressingtwo gray scales by using only the most significant 1-bit signal isreferred to as a second display mode.

Note that, in general, in the case of using an n-bit signal as the inputdigital video signal, the expression of gray scales using the n-bitsignal is referred to as the first display mode. The expression of grayscales using only m bits of the signal (where m is a natural number lessthan n) from among the n bits is referred to as the second display mode.

Note that the first bit of the n-bit digital image signal is taken asthe most significant bit, and that the n-bit is taken as the leastsignificant bit.

In the second display mode, gray scales are expressed without using thesignal corresponding to the lower bits of the digital image signal inthe first display mode.

One frame period is divided into four sub-frame periods SF1 to SF4. Thesub-frame periods SF1 to SF4 express in order the sub-frame periodcorresponding to the most significant bit to the sub-frame periodcorresponding to the least significant bit, and appear in this order tostructure one frame period.

In the first display mode, the gray scales are expressed using all ofthe input 4-bit digital video signal, and therefore the signal inputfrom the signal controller circuit to the source signal line drivercircuit is the same as the case of expressing gray scales using the4-bit digital image signal. Further, the clock pulse S_CLK and the startpulse S_SP for the source signal line driver circuit and the clock pulseG_CLK and the start pulse G_SP for the gate signal line driver circuitwhich are output from the display controller, are also the same as thoseused in the case of expressing gray scales using the 4-bit digital imagesignal.

A method of driving the display device in the second display mode isexplained below.

A timing chart showing the method of driving the display device in thesecond display mode is shown in FIG. 9.

Signals are input to respective pixels in the first sub-frame periodSF1. When the signals, are input to all of the pixels, the opposingelectric potential changes to have an electric potential difference fromthe electric power source electric potential so that the light emittingelements emit light. The light emitting elements of all of the pixelsare thus placed in a light emitting state or a non-light emitting state.

Operations in the first sub-frame period are the same as the operationsperformed in the first display mode.

Next, the digital image signal is also similarly written to all of thepixels in the write in period of the second sub-frame period. However,in the following display period, the electric potential of the opposingelectrode does not change so as to have an electric potential differencefrom the electric power source electric potential so that the lightemitting element emit light. That is, the light emitting elements of thepixels do not emit light in the display period of the second sub-frameperiod, regardless of the signals input to the pixels. This period isdenoted as non-display.

Operations in the second sub-frame period are similarly repeated in thethird sub-frame period and in the fourth sub-frame period to thuscomplete one frame period.

The period in which the pixels perform display during one frame periodis only the first sub-frame period. The number of times that the lightemitting elements of the pixels emit light can thus be lowered in thesecond display mode, and the electric power consumption of the displaydevice can be reduced.

In a conventional display device, each pixel of the display device doesnot perform display in a period except a sub-frame period which iscorresponding to an upper bit in switching to a second display mode forexpressing gray scales without using information of lower bits. However,in each driver circuit (source signal line driver circuit and gatesignal line driver circuit), write in operation of the digital videosignal to each pixel is performed. At this time, start pulses, clockpulses and the like are input to each driver circuit of the displaydevice to thereby continue the operation.

Therefore, even in the second display mode in which gray scale displayis performed with a small amount of information, each of the drivercircuits repeatedly performs sampling of the digital image signal, whichis the same as sampling operations in the first display mode. Electricpower is therefore consumed for sampling, and there is a problem thatthe electric power consumption cannot be made smaller.

Furthermore, in the sub-frame periods except the sub-frame period duringwhich display is actually performed, the pixels are all uniformly in anon-display state during which light is not emitted. There is thereforea problem that the proportion of the effective display period in oneframe period is small.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a display device inwhich electric power consumption is small and in which the proportionthat an effective display period occupies per one frame period is large,in the case of performing drive in which the number of gray scalesexpressed is reduced. In addition, an object of the present invention isto provide a method of driving the display device.

Write in of the lower significant bits of a digital video signal to amemory is eliminated by a memory controller of a signal control circuitin a display device during a second display mode as compared to a firstdisplay mode. Further, read out of the lower significant bits of thedigital signal from the memory is also eliminated. Each driver circuitthus inputs a digital image signal with a reduced amount of information(a second digital image signal) to a source signal line driver circuitin comparison to a digital image signal in the first display mode (afirst digital image signal). Corresponding to this operation, a displaycontroller functions to produce start pulses and clock pulses each witha lower frequency which are input to each of the driver circuits (thesource signal line driver circuit and a gate signal line drivercircuit). Write in periods and display periods of the sub-frame periodsparticipating in display can thus be set longer.

A display device in which the electric power consumption is small and inwhich the proportion that an effective display period occupies per oneframe period is large, can thus be provided in accordance with the abovestructure, as well as a method of driving the display device.

Structures of the present invention are discussed below.

In accordance with the present invention, a display device is provided,in which:

-   -   one, frame period is divided into a plurality of sub-frame        periods;    -   turn on or turn off is performed in the sub-frame periods, and        gray scales are expressed by the total amount of turn on time        within the one frame period:    -   there is a first display mode in which one frame period is        divided into n sub-frame periods (where n is a natural number),        and a second display mode in which one frame period is divided        into m sub-frame periods (where m is a natural number less than        n).

In accordance with the present invention, a display device is provided,and the display device has:

-   -   a display; and    -   a display controller for supplying a clock signal,    -   wherein one frame period is divided into a plurality of        sub-frame periods,    -   wherein turn on or turn off is performed in the sub-frame        periods, and gray scales are expressed by the total amount of        turn on time within the one frame period and    -   wherein the display controller supplies clock signals with        different frequencies to the display in accordance the number of        gray scales expressed.

In accordance with the present invention, a display device is provided,and the display device has a memory for storing one frame period ofdigital video signals,

-   -   wherein the one frame period is divided into a plurality of        sub-frame periods,    -   wherein turn on or turn off is performed in the sub-frame        periods, and gray scales are expressed by the total amount of        turn on time within the one frame period, and    -   wherein the digital video signals stored in the memory are read        out at different frequencies.

In accordance with the present invention, a display device is provided,and the display device has:

-   -   a display:    -   a display controller for supplying a clock signal; and    -   a memory for storing one frame period of digital video signals,    -   wherein one frame period is divided into a plurality of        sub-frame period,    -   wherein turn on or turn off is performed in the sub-frame        periods, and gray scales are expressed by the total amount of        turn on time within the one frame period, and    -   wherein the display controller supplies clock signals with        different frequencies to the display in accordance with the        number of gray scales expressed, and the digital video signals        stored in the memory are read out at different frequencies.

In accordance with the present invention, a display device is provided,and the display device has:

-   -   a display; and    -   a display controller for supplying a clock signal,    -   wherein one frame period is divided into a plurality of        sub-frame periods,    -   wherein turn on or turn off is performed in the sub-frame        periods, and gray scales are expressed by the total amount of        turn on time within the one frame period,    -   wherein there is a first display mode in which the one frame        period is divided into n sub-frame periods (where n is a natural        number), and a second display mode in which the one frame period        is divided into m sub-frame periods (where m is a natural number        less than n), and    -   wherein the display controller supplies a clock signal with a        different frequency to the display in the first display mode        from in the second display mode.

In accordance with the present invention, a display device is provided,the display device has a memory for storing one frame period of digitalvideo signals,

-   -   wherein the one frame period is divided into a plurality of        sub-frame periods,    -   wherein turn on or turn off is performed in the sub-frame        periods, and gray scales are expressed by the total amount of        turn on time within the one frame period,    -   wherein there is a first display mode in which the one frame        period is divided into n sub-frame periods (where n is a natural        number), and a second display mode in which the one frame period        is divided into m sub-frame periods (where m is a natural number        less than n); and    -   wherein the digital video signals stored in the memory are read        out at a different frequency in the first display mode from in        the second display mode.

In accordance with the present invention, a display device is provided,the display device has:

-   -   a display;    -   a display controller for supplying a clock signal; and    -   a memory for storing one frame period of digital video signals,    -   wherein one frame period is divided into a plurality of        sub-frame periods,    -   wherein turn on or turn off is performed in the sub-frame        periods, and gray scales are expressed by the total amount of        turn on time within the one frame period,    -   wherein there is a first display mode in which the one frame        period is divided into n sub-frame periods (where n is a natural        number), and a second display mode in which the one frame period        is divided into m sub-frame periods (where m is a natural number        less than n); and    -   wherein the display controller supplies a clock signal with a        different frequency to the display in the first display mode        from in the second display mode, and the digital video signals        stored in the memory are read out at a different frequency in        the first display mode from in the second display mode.

The display device may also be one in which the brightness during turnon in the sub-frame periods differs in accordance with the number ofgray scales expressed.

The display device may also be one in which the brightness during turnon in the sub-frame periods differs between the first display mode andthe second display mode.

In accordance with the present invention, a display device is provided,the display device has a display and a memory,

-   -   wherein the display has a plurality of pixels,    -   wherein each of the plurality of pixels has a light emitting        element,    -   wherein digital video signals are written to the memory,    -   wherein a digital image signal is output to the display from the        memory,    -   wherein one frame period is divided into a plurality of        sub-frame periods,    -   wherein each of the plurality of sub-frame periods has a write        in period for inputting the digital image signal to the        plurality of pixels, and a display period for placing the light        emitting elements into a light emitting state or a non-light        emitting state in accordance with the digital image signal input        to the plurality of pixels during the write in period,    -   wherein display of an image is performed by switching between: a        first display mode in which gray scales are expressed by using        first bit to n-th bit signals (where n is a natural number) of        the digital video signal; and a second display mode in which        gray scales are expressed by using first bit to m-th bit signals        (where m is a natural number less than n) of the digital video        signal,    -   wherein the first bit to n-th bit signals of the digital video        signal are stored in the memory in the first display mode, and        the first bit to the m-th bit signals of the digital video        signal are stored in the memory in the second display mode, and    -   wherein the write in period and the display period of the        sub-frame period corresponding to a t-bit (where t is a natural        number less than m) in the second display mode are respectively        longer than the write in period and the display period of the        sub-frame period corresponding to a t-bit in the first display        mode.

In accordance with the present invention, a display device is provided,the display device has a display and a memory,

-   -   wherein the display has a plurality of pixels,    -   wherein each of the plurality of pixels has a light emitting        element,    -   wherein digital video signals are written to the memory,    -   wherein a digital image signal is output to the display from the        memory,    -   wherein one frame period is divided into a plurality of        sub-frame periods,    -   wherein each of the plurality of sub-frame periods has a write        in period for inputting the digital image signal to the        plurality of pixels, and a display period for placing the light        emitting elements into a light emitting state or a non-light        emitting state in accordance with the digital image signal input        to the plurality of pixels during the write in period,    -   wherein display of an image is performed by switching_(—)        between: a first display mode in which gray scales are expressed        by using first bit to n-th bit signals (where n is a natural        number) of the digital video signal; and a second display mode        in which gray scales are expressed by using first to m-th bit        signals (where m is a natural number less than n) of the digital        video signal,    -   wherein n sub-frame periods exist in the first display mode,    -   wherein the ratio of lengths of display periods Ts1 to Tsn which        the n sub-frame periods respectively have is        2⁰:2⁻¹:2^(−(n−2)):2^(−(n−1)),    -   wherein m sub-frame periods exist in the second display mode,    -   wherein the ratio of lengths of display periods Ts1 to Tsm of        the which the m sub-frame periods respectively have is        2⁰:2⁻¹:2^(−(m−2):2^(−(m−1)),    -   wherein switching takes place between the first display mode in        which the first bit to the n-th bit signals of the digital video        signal are stored in the memory, and the second display mode in        which the first bit to the m-bit signals of the digital video        signal are stored in the memory, and    -   wherein the write in period and the display period of the        sub-frame period corresponding to a t-bit in the second display        mode are respectively longer than the write in period and the        display period of the sub-frame period corresponding to a t-bit        in the first display mode (where t is a natural number less than        m).

The display device may also be one in which the electric potential ofthe opposing electrode of the light emitting element is changed so thatthe brightness of light emitted from the light emitting element in alight emitting state becomes less in the display period corresponding tothe t-bit in the second display mode, than that in the display periodcorresponding to the t-bit in the first display mode.

In accordance with the present invention, a display device is provided,the display device has a signal control circuit, a display controller,and a display,

-   -   wherein the display has a source signal line driver circuit, a        gate signal line driver circuit, and a plurality of pixels,    -   wherein each of the plurality of pixels has a light emitting        element,    -   wherein the signal control circuit has a CPU, a memory, and a        memory controller,    -   wherein the display controller inputs a clock pulse for the        source signal line driver circuit and a start pulse for the        source signal line driver circuit to the source signal line        driver circuit, and inputs a clock pulse for the gate signal        line driver circuit and a start pulse for the gate signal line        driver circuit to the gate signal line driver circuit,    -   wherein digital video signals are written to the memory,    -   wherein a digital image signal is output from the memory to the        display,    -   wherein one frame period is divided into a plurality of        sub-frame periods,    -   wherein each of the plurality Of sub-frame periods has a write        in period for inputting the digital image signal to the        plurality of pixels, and a display period for placing the light        emitting elements into a light emitting state or a non-light        emitting state in accordance with the digital image signal input        to the plurality of pixels during the write in period,    -   wherein display of an image is performed by switching between a        first display mode in which gray scales are expressed by using        first bit to n-th bit signals (where n is a natural number) of        the digital video signal, and a second display mode in which        gray scales are expressed by first to a in-bit signals (where m        is a natural number less than n) of the digital video signal,    -   wherein, in the first display mode, the memory controller writes        the first bit to n-th bit of the digital video signal to the        memory from the CPU, and further, outputs the digital video        signal written into the memory to the source signal line driver        circuit as the digital image signal,    -   wherein, in the second display mode, the memory controller        writes the first bit to m-bit of the digital signal to the        memory from the CPU, and further, outputs the digital video        signal written into the memory to the source signal line driver        circuit as the digital image signal, and    -   wherein the display controller lowers frequency of each of the        clock pulse for the source signal line driver circuit, the start        pulse for the source signal line driver circuit, the clock pulse        for the gate signal line driver circuit, and the start pulse for        the gate signal line driver circuit in the second display mode        compared to those in the first display mode.

The display device may also be one in which:

-   -   the display controller has a variable frequency divider circuit;    -   a gray scale controller signal is input to the variable        frequency divider circuit; and    -   frequency of each of the clock pulse for the source signal line        driver circuit, the start pulse for the source signal line        driver circuit, the clock pulse for the gate signal line driver        circuit, and the start pulse for the gate signal line driver        circuit are made lower in the second display mode compared to        those in the first display mode.

The display device may also be one in which:

-   -   the display controller has an electric power source controller        circuit used for the light emitting element; and    -   the electric potential of the opposing electrode of the light        emitting element is changed so that the brightness of light        emitted from the light emitting element in a light emitting        state becomes less in the display period corresponding to the        t-bit in the second display mode, than that in the display        period corresponding to the t-bit in the first display mode        (where t is a natural number less than m).

The display device of the present invention may be used in videocameras, DVD playback devices, television receivers, heat mounteddisplays, portable information terminals, personal computers, and thelike.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are diagrams showing timing charts for a method ofdriving a display device of the present invention;

FIG. 2 is a diagram showing a structure of a memory controller of thedisplay device of the present invention;

FIG. 3 is a diagram showing a structure of a display controller of thedisplay device of the present invention;

FIG. 4 is a clock diagram showing a structure of the display device ofthe present invention;

FIGS. 5A and 5B are diagrams showing timing charts for a time gray scaledriving method;

FIG. 6 is a block diagram showing the structure of the display device ofthe present invention;

FIG. 7 is a diagram showing a structure of a pixel portion of thedisplay device;

FIG. 8 is a diagram showing a structure of a pixel of the displaydevice;

FIG. 9 is a diagram showing a timing chart of a conventional method ofdriving a display device

FIG. 10 is a block diagram showing a structure of the conventionaldisplay device;

FIG. 11 is a diagram showing a structure of a memory controller of theconventional display device;

FIG. 12 is a diagram showing a structure of a display controller of theconventional display device;

FIGS. 13A to 13C are diagrams showing a method of sealing a lightemitting element of the display device of the present invention;

FIGS. 14A to 14F are diagrams showing electronic equipment of thepresent invention;

FIG. 15 is a diagram showing a structure of a source signal line drivercircuit of the display device of the present invention; and

FIG. 16 is a diagram showing a structure of a gate signal line drivercircuit of the display device of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment mode of the present invention is explained.

Timing charts for a method of driving a display device of the presentinvention are shown in FIGS. 1A and 1B.

A display device into which a 4-bit digital video signal is input, isfocused upon in FIGS. 1A and 1B. A 4-bit digital image signal is inputto a display to perform display of an image in a first display mode. Ina second display mode, a gray scale is expressed by a 1-bit digitalimage signal, using only the most significant one bit of the digitalvideo signal from among the four bits of the digital video signal.Although an example is explained in the embodiment mode using theaforementioned case, a display device of the present invention is notlimited to this case.

In general, a display device into which an n-th bit digital video signal(where n is a natural number) is input, is focused upon. It is possibleto express 2^(n) gray scales by using, n sub-frame periods SF1 to SFnand the n-th bit digital image signal in the first display mode. On theother hand, 2^(m) gray scales are expressed by using an m-bit digitalimage signal (where m is a natural number less than n) in the seconddisplay mode in accordance with switch-over operation. The presentinvention can also be applied to such a case.

Further, a display device into which n-th bit digital video signal isinput (where n is a natural number) is more generally focused upon. Inthe first display mode, the n-th bit digital image signal is input, andit is possible to express w gray scales (where w is a natural number)using r sub-frame periods (where r is a natural number). On the otherhand, u gray scales (where u is a natural number smaller than w) areexpressed in the second display mode by using s sub-frame periods (wheres is a natural number less than r) and m-th bit the digital image signal(where m is a natural number less than n) in accordance with switch-overoperation. The present invention can also be applied to such a case.

A timing chart in a case of the first display mode, in which the 4-bitsignal is input and 2⁴ gray scales are expressed, is shown in FIG. 1A.

Each pixel is selected to be in a light emitting state or in a non-lightemitting state in a display period in each of sub-frame periods SF1 toSF4 structuring one frame period. An opposing electric potential is setto be nearly the same as an electric power source electric potentialduring write in periods, and is changed in the display periods so as tohave an electric potential difference from the electric power sourceelectric potential to an extent that light emitting elements will emitlight.

These operations are similar to the conventional example, and a detailedexplanation is therefore omitted.

A timing chart in a case of the second display mode for expressing grayscales using only the most significant one bit signal is shown in FIG.1B.

Compared to the first display mode shown in FIG. 1A, the write in periodand the display period are set longer, and one frame period roughlycorresponds to a first sub-frame period.

The structure of a display device for performing the aforementioneddriving operations is explained below.

A block diagram of the display device for performing the aboveoperations is shown in FIG. 4 and FIG. 6.

The display device is structured by a signal control circuit 101, adisplay controller 102, and a display 100.

The display controller 102 supplies a start pulse SP and a clock pulseCLK to the display 100.

The signal control circuit 101 is structured by a CPU 104, a memory A105, a memory B 106, and a memory controller 103.

An example of a display device is shown in FIG. 4 into which the 4-bitdigital video signal is input, and which expresses gray scales using the4-bit digital image signal in the first display mode. The memory A 105is structured by memories 105_1 to 105_4 for storing a first bit to afourth bit, respectively, of the digital video signal. Similarly, thememory B 106 is structured by memories 106_1 to 106_4 for storing afirst bit to a fourth bit, respectively, of the digital video signal.The memories corresponding to each bit of the digital signal each have aplurality of memory elements capable of storing one bit of the signal asmany as the number of pixels structuring one screen.

In general, the memory A is structure by memories 105_1 to 105_n forstoring a first bit to a n-th bit of information, respectively, in adisplay device which is capable of expressing gray scales by using ann-th bit digital image signal. Similarly, the memory B is structure bymemories 106_1 to 106_n for storing the first bit to the n-th bit ofinformation, respectively. The memories corresponding to each bit ofinformation each have a capacity that is capable of storing one bit ofthe signal as many as the number of pixels structuring one screen.

The structure of the memory controller 103 of FIG. 4 is shown in FIG. 2.

The memory controller 103 is structured by a gray scale limiter circuit201, a memory R/W circuit 202, a standard oscillator circuit 203, avariable frequency divider circuit 204, an x-counter 205 a, a y-counter205 b, an x-decoder 206 a, and a y-decoder 206 b in FIG. 2.

The above-described memory A and the memory B are both taken togetherand denoted as memory. Furthermore, the memory is structured by aplurality of memory elements. The memory elements are selected by using(x,y) addresses.

A signal from the CPU 104 is input to the memory R/W circuit 202 throughthe gray scale limiter circuit 201. The gray scale limiter circuit 201inputs the signal to the memory R/W circuit 202 in accordance witheither the first display mode or the second display mode. The memory R/Wcircuit 202 selects whether or not to write the digital video signalcorresponding to each bit into the memory, in accordance with the signalfrom the gray scale limiter circuit 201. Similarly, the digital signalwritten into the memory is selected in read out operation.

Further, the signal from the CPU 104 is input to the standard oscillatorcircuit 203. A signal from the standard oscillator circuit 203 is inputto the variable frequency divider circuit 204, and converted to a signalwith a suitable frequency. A signal from the gray scale limiter circuit201 is input to the variable frequency divider circuit 204, inaccordance with either the first display mode or the second displaymode. Based on the input signal, a signal from the variable frequencydivider circuit 204 selects, the x-address of the memory, through thex-counter 205 a and the x-decoder 206 a. Similarly, a signal from thevariable frequency divider circuit 204 is input to the y-counter 205 band to the y-decoder 206 b, and selects the y-address of the memory.

The amount of information for the signal written into the memory and forthe signal (digital image signal) output from the memory, taken from thedigital video signal input to the signal controller circuit, can becontrolled by using memory controller 103 with the above structure inthe case where high level gray scale display is not necessary. Further,the frequency for reading out the signal from the memory can be changed.

The above describes the memory controller 103.

Further, the structure of the display controller 102 in FIG. 4 isexplained.

FIG. 3 is a diagram showing the structure of the display controller ofthe present invention.

The display controller 102 is structured by a standard clock generatorcircuit 301, a variable frequency divider circuit 302, a horizontalclock generator circuit 303, a vertical clock generator circuit 304, andan electric power source 305 used for the light emitting elements.

A clock signal 31 input from the CPU 104 is input to the standard clockgenerator circuit 301, and a standard clock is generated. The standardclock is input to the horizontal clock generator circuit 303 and to thevertical clock generator circuit 304, through the variable frequencydivider circuit 302. A gray scale control signal 34 is input to thevariable frequency divider circuit 302. The frequency of the standardclock is changed in accordance with the gray scale control sismal 34.

The extent that the frequency of the standard clock is changed in thevariable frequency divider circuit 302 can be suitably determined by theoperator. This is because the sub-frame periods in the first displaymode, which are corresponding to the bits participating in theexpression of gray scales in the second display mode, differ inaccordance with their occupied proportion to one frame period.

In other words, in the second display mode, the sub-frame periods withinone frame period are cut with respect to the first display mode. Withthe present invention the effective display period within one frameperiod is set longer even in the second display mode, and therefore thestandard clock frequency is changed in the variable frequency dividercircuit 302. The percentage of the change in the frequency can bedetermined in accordance with the proportion of bits removed.

Further, a horizontal period signal 32 which determines a horizontalperiod is input to the horizontal clock circuit 303 from the CPU 104,and the clock pulse S_CLK and the start pulse S_SP for the source signalline driver circuit are output. Similarly, a vertical period signal 33which determines a vertical period is input to the vertical clockcircuit 304 from the CPU 104, and the clock pulse G_CLK and the startpulse G_SP for the gate signal line driver circuit are output.

The above explains the display controller 102.

The display device of the present invention thus eliminates read out ofthe less significant bits of the signal from the memory in the memorycontroller of the signal controller circuit during the second displaymode. Further, the frequency for reading out signals from the memory ismade smaller. Corresponding to these operations, the display controllerlowers the frequency of the sampling pulse SP and the frequency of theclock pulse CLK input to each of the driver circuits (the source signalline driver circuit and the gate signal line driver circuit), andlengthens the write in period and the display period of the sub-frameperiod for expressing the image.

For example, one frame period is divided into four sub-frame periods inthe first display mode. With the ratio of the display periods Ts1, Ts2,Ts3, and Ts4 of the respective sub-frame periods set to be 2⁰::2⁻¹::2⁻²::2 ⁻³, a display device for expressing 2⁴ gray scales usinga 4-bit digital image signal is considered. For simplicity, the lengthsof the display periods Ts1 to Ts4 of each sub-frame period are taken tobe 8, 4, 2, and 1, respectively. Further, the lengths of the write inperiods Ta1 to Ta4 of each sub-frame period are taken to be 1.Furthermore, a case of expressing gray scales using the most significantbit of the signal in the second display mode is considered.

The occupied proportion per one frame period by the sub-frame period inthe first display mode, that corresponds to the bit participating ingray scale expression in the second display mode, becomes 9/19.

In other words, the sub-frame period participating in gray scaleexpression in the second display mode is the sub-frame period (denotedby the reference symbol SF1) corresponding to the most significant bit.The occupied proportion per one frame period by SF1 in the first displaymode is 9/19.

If the structure of the present invention is not used, for example, as acase of using the conventional driving method shown in FIG. 9, 10/19 ofone frame period becomes the period which is not participating indisplay.

On the other hand, in accordance with the structure of the presentinvention, the frequency of the clock signal or the like input to eachdriver circuit of the display is changed in the second display mode, andthe write in period is set to have a length that is 19/9 times thelength of the write in period in the first display mode. Similarly, thedisplay period is also set to have a length that is 19/9 times thelength of the display period Ts1 of the sub-frame period SF1 which iscorresponding to the most significant bit in the first display mode. Thesub-frame period Sf1 can thus be made to occupy one frame period. Theperiods which do not participate in display during one frame period canthus be reduced in the second display mode.

In general, a display device which has a first display mode forexpressing gray scales using a first bit to a n-th bit signal (where nis a natural number), and has a second display mode for expressing grayscales using the first bit to a m-th bit signal (where m is a naturalnumber less than n) is focused upon.

The occupied proportion per one frame period by the sub-frame periods inthe first display mode, that correspond to the bits participating ingray scale expression in the second display mode, becomes 1/q (where qis a number greater than 1).

In other words, a case, in which the occupied proportion per one frameperiod by the sub-frame periods corresponding to the first bit to them-th bit is 1/q (where q is a number greater than 1) in the first mode,is considered.

In a sub-frame period corresponding to a t-th bit in the second displaymode (where t is a natural number less than or equal to m), thefrequency of each signal (such as clock pulses and start pulses) inputto each driver circuit of the display (the source signal line drivercircuit and the gate signal line driver circuit) is changed to be 1/qtimes its original value, and a write in period with a length that is qtimes the length of the write in period of the sub-frame periodcorresponding to the t-th bit in the first display mode is set.Similarly, in accordance with setting the length of the display periodto be q times the length of the display period of the sub-frame periodcorresponding to the t-th bit in the first display mode (where t is anatural number less than or equal to m), display of an image can also beperformed by sufficient use of one frame period.

The display period per one frame period of the light emitting elementcan thus also be made increased in the second display mode.

Therefore, in the second display mode, the brightness of the lightemitting element selected to be in a light emitting state in the displayperiod of the sub-frame period corresponding to the most significant bitcan be made smaller compared to the brightness of the light emittingelement selected to be in a light emitting state in the display periodof the sub-frame period corresponding to the most significant bit -inthe first display mode. Consequently, the voltage applied between ananode and a cathode of the light emitting element can be set lower inthe display periods with the second display mode.

A method of changing the voltage applied between the anode and thecathode of the light emitting element in accordance with the displaymode, is explained.

During the write in period, the electric power source control circuit305 in FIG. 3 used for the light emitting element maintains the electricpotential of the opposing electrode (opposing electric potential) of thelight emitting element at an electric potential which is nearly the sameas the electric power source electric potential. In the display period,the electric potential of the opposing electrode of the light emittingelement is controlled so as to have an electric potential differencefrom the electric power source electric potential to an extent that thelight emitting element will emit light. The gray scale control signal 34is also input to the electric power source control circuit 305 here. Theelectric potential of the opposing electrode of the light emittingelement is thus changed in order that the voltage applied between bothelectrodes of the light emitting element becomes smaller by an amountthat the light emitting period for the light emitting element becomeslonger.

In general, a case is considered, in which the display period of asub-frame period corresponding to the t-th bit in the second displaymode (where t is a natural number less than or equal to m) is set tohave a length that is q times the length of the display period of thesub-frame period corresponding to the t-th bit in the first display mode(where q is a number larger than 1). The brightness of the lightemitting element selected to be in a light emitting state in thesub-frame period corresponding to the t-th bit in the second displaymode can be set equal to 1/q times the brightness of the light emittingelement selected to be in a light emitting state in the sub-frame periodcorresponding to the t-th bit in the first display mode.

The voltage applied between both the electrodes of the light emittingelement can be made smaller in the second display mode, and thereforestress on the light emitting element due to the applied voltage can alsobe made smaller.

Note that although the display device shown is one which switchesbetween the first display mode and the second display mode, the presentinvention can also be applied to a case in which, in addition to thefirst display mode and the second display mode, at least one more modein which the number of gray scales expressed is changed are additionallyestablished, and display is performed by switching between the pluralityof modes.

Pixels with the structure shown in FIG. 8 in the conventional examplecan be used here to structure the pixel portion of the display of thedisplay device according to the present invention. Further, pixels withanother known structure can also be freely used.

For example, the following two types of pixels can be applied. The firsttype is a pixel in which the brightness of a light emitting element isdetermined by setting the voltage applied between the anode and thecathode of the light emitting element. The pixel with the structureshown in FIG. 8 corresponds to this type. The second type is a pixel inwhich the brightness of the light emitting element is determined bysetting the electric current flowing through the light emitting element.

Furthermore, circuits with known structures can be freely used for thesource signal line driver circuit and the gate signal line drivercircuit of the display of the display device according to the presentinvention.

In addition, it is also possible to apply the present invention not onlyto a display device using OLED elements, but also to self-light emittingtype display devices using FDPs, PDPs, and the like as light emittingelements.

Embodiments

Embodiments of the present invention are explained below.

Embodiment 1

An example of the structure of a source signal line driver circuit of adisplay device according to the present invention is explained inEmbodiment 1.

FIG. 15 shows an example of the structure of a source signal line drivercircuit.

The source signal line driver circuit is structured by a shift register,a scanning direction switching circuit, an LAT (A) and an LAT (B). Notethat, although only an LAT (A) portion 2612 and an LAT (B) portion 2618which are corresponding to one of outputs from the shift register areshown in FIG. 15, the LAT (A) and the LAT (B) correspond to all of theoutputs from the shift register using a similar structure.

A shift register 2601 is structure clocked inverters 2602 and 2603, aninverter 2604, and a NAND 2607. A start pulse S_SP for the source signalline driver circuit is input to the shift register 2601. By changing thestate of the clocked inverters 2602 and 2603 between a conductive stateand a non-conductive state in accordance with a clock pulse S_CLK forthe source signal line driver circuit and an inverted clock pulse S_CLKBfor the source signal line driver circuit which has an inverse polarityto that of the clock pulse S_CLK, sampling pulses are output in orderfrom the NAND 2607 to the LAT (A).

Further, the scanning direction switching circuit is structure by aswitch 2605 and a switch 2606, and works to switch the operationdirection of the shift register between left and right directions. InFIG. 15, the shift register outputs sampling pulses in order from theleft to the right in the case in which a left and right switching signalL/R corresponds to a Lo signal. On the other hand, if the left and rightswitching signal L/R is a Hi signal, then sampling pulses are output inorder from the right to the left.

Each stage of an LAT (A) 2613 is structured by clocked inverters 2614and 2615, and inverters 2616 and 2617.

The term “each stage of the LAT (A)” denotes an LAT (A) for taking in animage signal input to one source signal line here.

A digital image signal VD output from the signal control circuitexplained in the embodiment mode is input in p divisions (where p is anatural number) here. That is, signals corresponding to output to psource signal lines are input in parallel. If a sampling pulse is inputat the same time to the clocked inverters 2614 and 2615 of p stages ofthe LAT (A) 2612 through buffers 2608 to 2611, then the respective inputsignals in p divisions are sampled simultaneously in p stages of the LAT(A) 2612.

A source signal line driver circuit 2600 for outputting signal currentsto x source signal lines is explained here, and therefore x/p samplingpulses are output in order from the shift register per one horizontalperiod. The p stages of the LAT (A) 2613 simultaneously sample thedigital image signals which are corresponding to output to the p sourcesignal lines in accordance with each sampling pulse.

A read in method, in which the digital image signals thus input to thesource signal line driver circuit are divided into parallel signals of pphases and the p digital images signals are taken in by using onesampling pulse, is referred to as p-division drive in thisspecification.

A margin can be given to the sampling of the shift register in thesource signal line driver circuit by performing the above-stateddivision drive. The reliability of the display device can thus beincreased.

When all of the signals for one horizontal period are input to eachstage of the LAT (A) 2613, a latch pulse LP and an inverted latch pulseLPB which has a inverse polarity to the latch pulse LP are input, andthe signals input to each stage of the LAT (A) 2613 are all outputsimultaneously to each stage of an LAT (B) 2619.

Note that the term “each stage of the LAT (B)” used here denotes an LAT(B) circuit to which the signal from each stage of the LAT (A) is input.

The LAT (B) 2619 is structured by clocked inverters 2620 and 2621, andinverters 2622 and 2623. The signal output from the LAT (A) 2613 isstored in the LAT (B) and at the same time is output to each of sourcesignal lines S1 to Sx.

Note that, although not shown in the figures, circuits such as levelshifters and buffers may also be suitably formed.

Signals such as the start pulse S_SP and the clock pulse S_CLK, input tothe shift register, the LAT (A), and the LAT (B), are input from thedisplay controller shown in the embodiment mode of the presentinvention.

With the present invention, operations for inputting a digital imagesignal with a small number of bits to the LAT (A) of the source signalline driver circuit are performed by the signal controller circuit. Atthe same time, operations for reducing the frequency of the clock pulseS_CLK, the start pulse S_SP, and the like, input to the shift registerof the source signal line driver circuit, are performed by the displaycontroller.

Operations for sampling the digital image signal by the source signalline driver circuit can thus be reduced in the second display mode, andthe electric power consumption of the display device can be curbed.

Note that the source signal line driver circuit of the display deviceaccording to the present invention is not limited to the structure ofthe source signal line driver circuit of Embodiment 1, and that sourcesignal line driver circuits with known structure can also be freelyused.

Embodiment 2

An example of a structure of a gate signal line driver circuit of adisplay device according to the present invention is explained inEmbodiment 2.

The gate signal line driver circuit is structured by a shift register, ascanning direction switching circuit, and the like. Note that, althoughnot shown in the figure, circuits such as level shifters and buffers mayalso be suitably formed.

Signals such as a start pulse G_SP and a clock pulse G_CLK are input tothe shift register, and a gate signal line selection signal is output.

The structure of the gate signal line driver circuit is explained usingFIG. 16.

A shift register 3601 is structured by clocked inverters 3602 and 3603,an inverter 3604, and a NAND 3607. The start pulse G_FSP is input to theshift register 3601. By changing the state of the clocked inverters 3602and 3603 between a conductive state and a non-conductive state inaccordance with a clock pulse G_CLK and an inverted clock pulse G_CLKBwhich has a inverse polarity to the clock pulse G_CLK, sampling pulsesare output in order from the NAND 3607.

Further, the scanning direction switching circuit is structure by aswitch 3605 and a switch 3606, and functions to switch the operationdirection of the shift register between left and right directions. InFIG. 16, the shift register outputs sampling pulses in order from theleft to the right in the case in which a left and right switching signalU/D corresponds to a Lo signal. On the other hand, if the left and rightswitching signal U/D is a Hi signal, then sampling pulses are output inorder from the right to the.

The sampling pulses output from the shift register are input to a NOR3608, and operation is performed with an enable signal ENB. Thisoperation is performed in order to prevent a condition in which adjacentgate signal lines are selected at the same time due to dull samplingpulses. The signals output from the NOR 3608 are output to gate signallines G1 to Gy, through buffers 3609 and 3610.

Note that, although not shown in the figure, level shifters and buffersmay also be formed as appropriate.

Signals such as the start pulse G_SP and the clock pulse G_CLK input tothe shift register are input from a display controller shown in theembodiment mode.

With the present invention, operations to reduce the frequency of theclock pulse G_CLK, the start pulse G_SP, and the like, input to theshift register of the gate signal line driver circuit, are performed bythe display controller in the second display mode.

Sampling operations of the gate signal line driver circuit can thereforebe reduced, and the electric power consumption of the display device canthus be curbed, in the second display mode.

Note that the gate signal line driver circuit of the display deviceaccording to the present invention is not limited to the structure ofthe gate signal line driver circuit of Embodiment 2. Gate signal linedriver circuits with known structures can be freely used.

It is possible to freely combine Embodiment 2 with Embodiment 1.

Embodiment 3

In this embodiment, a method of sealing the display device of thepresent invention is described with reference to FIGS. 13A to 13C.

FIG. 13A is a top view of a display device, FIG. 13B is a sectional viewtaken along a line A-A′ of FIG. 13A, and FIG. 13C is a sectional viewtaken along a line B-B′ of FIG. 13A.

A seal member 4009 is provided so as to surround a pixel portion 4002, asource signal line driver circuit 4003, and first and second gate signalline driver circuits 4004 a and 4004 b, which are provided on asubstrate 4001. Further, a sealing member 4008 is provided over thepixel portion 4002, the source signal line driver circuit 4003, and thefirst and the second gate signal line driver circuits 4004 a and 4004 b.Thus, the pixel portion 4002, the source signal line driver circuit4003, and the first and the second gate signal line driver circuits 4004a and 4004 b are sealed with a filler 4210 and by the substrate 4001,the seal member 4009, and the sealing member 4008.

Further, the pixel portion 4002, the source signal line driver circuit4003, and the first and the second gate signal line driver circuits 4004a and 4004 b provided on the substrate 4001 include a plurality of TFTs.FIG. 13B typically shows driving TFTs (here, an n-channel TFT and ap-channel TFT are shown) 4201 included in the source signal line drivercircuit 4003 and a driving TFT 4202 included in the pixel portion 4002,which are formed on an under film 4010.

In Embodiment 3, the p-channel TFT or the n-channel TFT fabricated by awell-known method are used as the driving TFTs 4201, and a p-channel TFTfabricated by a well-known method is used as the driving TFT 4202. Thestorage capacitor (not illustrated) connected to the gate of the drivingTFT 4202 is provided in the pixel portion 4002.

An interlayer insulating film (flattening film) 4301 is formed on thedriving TFTs 4201 and the driving TFT 4202, and a pixel electrode(anode) 4203 electrically connected to a drain region of the driving TFT4202 is formed thereon. A transparent conductive film having a high workfunction is used as the pixel electrode 4203. A compound of indium oxideand tin oxide, a compound of indium oxide and zinc oxide, zinc oxide,tin oxide, or indium oxide can be used for the transparent conductivefilm. Further, the transparent conductive film added with gallium may beused.

An insulating film 4302 is formed on the pixel electrode 4203, and anopening portion is formed in the insulating film 4302 over the pixelelectrode 4203. In this opening portion, an organic compound layer 4204is formed on the pixel electrode 4203. A well-known organic material orinorganic material can be used for the organic compound 4204. Althoughthe organic material includes a low molecular system (monomer system)and a high molecular system (polymer system), either may be used.

As a formation method of the organic compound layer 4204, a well-knownevaporation technique or coating technique may be used. The structure ofthe organic compound layer may be a laminate structure obtained byfreely combining a hole injection layer, a hole transfer layer, a lightemitting layer, an electron transfer layer, or an electron injectionlayer, or a single layer structure.

A cathode 4205 made of a conductive film (typically, a conductive filmcontaining aluminum, copper or silver as its main ingredient, or alaminate film of those and another conductive films) having a lightshielding property is formed on the organic compound layer 4204. It isdesirable that moisture and oxygen existing on the interface between thecathode 4205 and the organic compound layer 4204 are removed to theutmost. Accordingly, it is necessary to make such contrivance that theorganic compound layer 4204 is formed in a nitrogen or rare gasatmosphere, and the cathode 4205 is formed while the organic compoundlayer is not exposed to oxygen or moisture. In this embodiment, amulti-chamber system (cluster tool system) film forming apparatus isused, so that the film formation as described above is enabled. Apredetermined voltage is applied to the cathode 4205.

In the manner as described above, a light emitting element 4303constituted by the pixel electrode (anode) 4203, the organic compoundlayer 4204, and the cathode 4205 is formed. Then, a protection film 4209is formed on the insulating film 4302 so as to cover the light emittingelement 4303. The protection film 4209 is effective to prevent oxygen,moisture and the like from penetrating into the light emitting element4303.

Reference numeral 4005 a designates a drawing wiring line connected to apower supply line and is electrically connected to a source region ofthe driving TFT 4202. The drawing wiring line 4005 a passes between theseal member 4009 and the substrate 4001, and is electrically connectedto an FPC wiring line 4301 included in an FPC 4006 through ananisotropic conductive film 4300.

As the sealing member 4008, a glass member, a metal member (typically, astainless member), a ceramic member, or a plastic member (including aplastic film) can be used. As the plastic member, an FRP(Fiberglass-Reinforced Plastics) plate, a PVF (polyvinyl fluoride) film,a Mylar film, a polyester film or an acryl resin film can be used.Further, a sheet having such a structure that an aluminum foil isinterposed between PVF films or Mylar films can also be used.

However, in the case where the radiation direction of light from thelight emitting element is directed toward the side of a cover member,the cover member must be transparent. In this case, a transparentmaterial such as a glass plate, a plastic plate, a polyester film, or anacryl film is used.

As the filler 4103, in addition to an inert gas such as nitrogen orargon; ultraviolet ray curing resin or thermosetting resin can be used,and PVC (polyvinyl chloride), acryl, polyimide, epoxy resin, siliconeresin, PVB (polyvinyl butyral), or EVA (ethylene-vinyl acetate) can beused. In this embodiment, nitrogen was used as the filler.

Further, in order to expose the filler 4210 to a hygroscopic material(preferably, barium oxide) or a material capable of adsorbing oxygen, arecess portion 4007 is provided on the surface of the sealing member4008 at the side of the substrate 4001 and the hygroscopic material orthe material 4207 capable of adsorbing oxygen is disposed. Then, inorder to prevent the hygroscopic material or the material 4207 capableof adsorbing oxygen from scattering, the hygroscopic material or thematerial capable of adsorbing oxygen are held in the recess portion 4007by a recess cover member 4208. Note that, the recess cover member 4208is formed into a fine mesh, and has such a structure that air ormoisture is permeated and the hygroscopic material or the material 4207capable of adsorbing oxygen is not-permeated. The deterioration of thelight emitting element 4303 can be suppressed by providing therewith thehygroscopic material or the material 4207 capable of adsorbing oxygen.

As shown in FIG. 13C, at the same time as the formation of the pixelelectrode 4203, a conductive film 4203 a is formed to be in contact withthe drawing wiring line 4005 a.

The anisotropic conductive film 4300 includes a conductive filler 4300a. The substrate 4001 and the FPC 4006 are thermally compressed, so thatthe conductive film 4203 a on the substrate 4001 and the FPC wiring line4301 on the FPC 4006 are electrically connected through the conductivefiller 4300 a.

It is possible to freely combine Embodiments 1 and 2 with Embodiment 3.

Embodiment 4

This embodiment describes electronic equipment of the present inventionwith reference to FIGS. 14A to 14F.

FIG. 14A is a schematic diagram of a portable information terminalaccording to the present invention. The portable information terminal iscomposed of a main body 2701 a, operation switches 2701 b, a powerswitch 2701 c, an antenna 2701 d, a display unit 2701 e, and an externalinput port 2701 f. A display device having the structure shown inEmbodiment Modes and Embodiments 1 through 3 is used in the display unit2701 e.

FIG. 14B is a schematic diagram of a personal computer according to thepresent invention. The personal computer is composed of a main body 2702a, a case 2702 b, a display unit 2702 c, operation switches 2702 d, apower switch 2702 e, and an external input port 2702 f. A display devicehaving the structure shown in Embodiment Modes and Embodiments 1 through3 is used in the display unit 2702 c.

FIG. 14C is a schematic diagram of an image reproducing device accordingto the present invention. The image reproducing device is composed of amain body 2703 a, a case 2703 b, a recording medium 2703 c, a displayunit 2703 d, an audio output unit 2703 e; and operation switches 2703 f.A display device having the structure shown in Embodiment Modes andEmbodiments 1 through 3 is used in the display unit 2703 d.

FIG. 14D is a schematic diagram of a television according to the presentinvention. The television is composed of a main body 2704 a, a case 2704b, a display unit 2704 c, and operation switches 2704 d. A displaydevice having the structure shown in Embodiment Modes and Embodiments 1through 3 is used in the display unit 2704 c.

FIG. 14E is a schematic diagram of a head mounted display according tothe present invention. The head mounted display is composed of a mainbody 2705 a, a monitor unit 2705 b, a headband 2705 c, a display unit2705 d, and an optical system 2705 e. A display device having thestructure shown in Embodiment Modes and Embodiments 1 through 3 is usedin the display unit 2705 d.

FIG. 14F is a schematic diagram of a video camera according to thepresent invention. The video camera is composed of a main body 2706 a, acase 2706 b, a connection unit 2706 c, an image receiving unit 2706 d,an eye piece unit 2706 e, a battery 2706 f, an audio input unit 2706 g,and a display unit 2706 h. A display device having the structure shownin Embodiment Modes and Embodiments 1 through 3 is used in the displayunit 2706 h.

Not limited to the above-mentioned electronic equipments, the presentinvention can be applied to various electronic equipments.

The electric power consumption of a display device can be reduced withthe aforementioned structures of the present invention. In addition, itbecomes possible to lengthen the display period in one frame period,even in the case in which the number of sub-frame periods used forexpressing gray scales is reduced. Thus, it becomes possible to providea display device which is capable of clear image display, and to providea method of driving the display device.

Furthermore, the display period for a light emitting element per oneframe period can be increased, and therefore the voltage applied betweenan anode and a cathode of the light emitting element can be set lower inthe case of expressing the same brightness per frame. It thus becomespossible to provide a display device with high reliability.

It is also possible to apply the present invention to self-lightemitting type display devices using FDPs, PDPs, and the like, not onlyto display devices using OLED elements.

1. A semiconductor device comprising: a first substrate; a pixel portioncomprising a light-emitting element over the first substrate, whereinthe light-emitting element comprises an organic compound layerinterposed between a first electrode and a second electrode, the organiccompound layer comprising a light-emitting layer; a seal member thatsurrounds the pixel portion; a hygroscopic material that overlaps with aregion surrounded by the seal member and does not overlap with the pixelportion; and wherein a gray scale of a pixel in the pixel portion ischanged in accordance with a time in which the light-emitting elementemits light.
 2. The semiconductor device according to claim 1, whereinthe pixel comprises a transistor electrically connected to thelight-emitting element.
 3. The semiconductor device according to claim1, further comprising a driver circuit electrically connected to thepixel, wherein the driver circuit overlaps with the hygroscopicmaterial.
 4. The semiconductor device according to claim 1, furthercomprising: a signal line driver circuit electrically connected to thepixel portion, and a display controller electrically connected to thesignal line driver circuit, wherein the display controller comprises avariable frequency divider circuit.
 5. A display module comprising thesemiconductor device according to claim 1, wherein the display modulecomprises a flexible printed circuit electrically connected to a wiringon the first substrate, and wherein the wiring extends to the regionsurrounded by the seal member from outside the region surrounded by theseal member.
 6. A portable information terminal comprising the displaymodule according to claim
 5. 7. A semiconductor device comprising: afirst substrate; a pixel portion comprising an interlayer insulatingfilm and a light-emitting element over the interlayer insulating film,wherein the light-emitting element comprises: a first electrode over theinterlayer insulating film; an insulating film over the first electrode,wherein the insulating film has an opening that overlaps with the firstelectrode; an organic compound layer over the first electrode and theinsulating film, the organic compound layer comprising a light-emittinglayer; and a second electrode over the organic compound layer and theinsulating film; a seal member that surrounds the pixel portion; ahygroscopic material that overlaps with a region surrounded by the sealmember and does not overlap with the pixel portion; and a secondsubstrate fixed to the first substrate with the seal member withoutinterposing the insulating film between the seal member and the firstsubstrate; wherein the second substrate comprises a recess region onwhich the hygroscopic material is disposed, wherein a gray scale of apixel in the pixel portion is changed in accordance with a time in whichthe light-emitting element emits light.
 8. The semiconductor deviceaccording to claim 7, wherein an element is provided between theinterlayer insulating film and the first substrate, wherein theinterlayer insulating film is a flattening film that comprises a firstportion overlapping with the element and a second portion notoverlapping with the element, and wherein a height of the first portionand a height of the second portion are substantially the same with eachother.
 9. The semiconductor device according to claim. 8, wherein theelement is a transistor.
 10. The semiconductor device according to claim7, further comprising a driver circuit electrically connected to thepixel, wherein the driver circuit overlaps with the hygroscopicmaterial.
 11. The semiconductor device according to claim 7, furthercomprising: a signal line driver circuit electrically connected to thepixel portion, and a display controller electrically connected to thesignal line driver circuit, wherein the display controller comprises avariable frequency divider circuit.
 12. A display module comprising thesemiconductor device according to claim 7, wherein the display modulecomprises a flexible printed circuit electrically connected to a wiringon the first substrate, and wherein the wiring extends to the regionsurrounded by the seal member from outside the region surrounded by theseal member.
 13. A portable information terminal comprising the displaymodule according to claim
 12. 14. A semiconductor device comprising: afirst substrate; a pixel portion comprising an interlayer insulatingfilm and a light-emitting element over the interlayer insulating film,wherein the light-emitting element comprises: a first electrode over theinterlayer insulating film, the first electrode comprising a compoundthat comprises indium, tin, and oxygen; an insulating film over thefirst electrode, wherein the insulating film has an opening thatoverlaps with the first electrode; an organic compound layer over thefirst electrode and the insulating film, the organic compound layercomprising a light-emitting layer; and a second electrode over theorganic compound layer and the insulating film, the second electrodecomprising aluminum; a seal member that surrounds the pixel portion; ahygroscopic material that overlaps with a region surrounded by the sealmember and does not overlap with the pixel portion; and a secondsubstrate fixed to the first substrate with the seal member withoutinterposing the insulating film between the seal member and the firstsubstrate; wherein the second substrate comprises a first regionoverlapping with the seal member and a second region on which thehygroscopic material is disposed, the first region being thicker thanthe second region, wherein a gray scale of a pixel in the pixel portionis changed in accordance with a time in which the light-emitting elementemits light.
 15. The semiconductor device according to claim 14, whereinan element is provided between the interlayer insulating film and thefirst substrate, wherein the interlayer insulating film is a flatteningfilm that comprises a first portion overlapping with the element and asecond portion not overlapping with the element, and wherein a height ofthe first portion and a height of the second portion are substantiallythe same with each other,
 16. The semiconductor device according toclaim 15, wherein the element is a transistor.
 17. The semiconductordevice according to claim 14, further comprising a driver circuitelectrically connected to the pixel, wherein the driver circuit overlapswith the hygroscopic material.
 18. The semiconductor device according toclaim 14, further comprising: a signal line driver circuit electricallyconnected to the pixel portion, and a display controller electricallyconnected to the signal line driver circuit, wherein the displaycontroller comprises a variable frequency divider circuit.
 19. A displaymodule comprising the semiconductor device according to claim 14,wherein the display module comprises a flexible printed circuitelectrically connected to a wiring on the first substrate, and whereinthe wiring extends to the region surrounded by the seal member fromoutside the region surrounded by the seal member.
 20. A portableinformation terminal comprising the display module according to claim19.